This invention generally relates to the heat treatment of metallic conduits, and is specifically concerned with the simultaneous heat treatment of a number of non-adjacent heat exchanger tubes in a steam generator.
Systems and methods for heat treating the heat exchanger tubes in nuclear steam generators are known in the prior art. For example, U.S. Pat. No. 4,820,359 assigned to the Westinghouse Electric Corporation discloses and claims a method for the heat treatment of the U-bends in such heat exchanger tubes. Still another example of known heat treatment systems and methods is disclosed in U.S. Pat. No. 4,816,089 which is also assigned to the Westinghouse Electric Corporation. In this last patent (the entire disclosure of which is incorporated herein by reference), a heater probe is oscillated within the section of a heat exchanger tube that is circumscribed by a support plate in order to relieve stresses generated by the accumulation of sludge and other debris in the annular space between the tube and the bore in the support plate through which the tube extends. Devices for heat treating a number of heat exchanger tubes simultaneously are also known in the art. Such a device is disclosed and claimed in U.S. patent application Ser. No. 213,923 filed June 30, 1988 by Paolo R. Zafred and also assigned to the Westinghouse Electric Corporation. This device is capable of implementing the stress relief process disclosed and claimed in the aforementioned U.S. Pat. No. 4,816,089.
While such prior art devices and methods work very well for their intended applications, recent refinements in the maintenance of heat exchanger tubes have created a need for a device capable of quickly and reliably heat treating a number of non-adjacent heat exchanger tubes simultaneously. Specifically, the Westinghouse Electric Corporation has recently pioneered new ways to reinforce damaged or cracked heat exchanger tubes by welding metal sleeves within these tubes, thereby creating a "hydraulic bridge" across the damaged sections thereof. While the installation of such sleeves by welding or braising techniques has been known for some time in the prior art, the new techniques developed by Westinghouse involve the use of a small, rotating mirror to direct a laser beam against the inner diameter of such a sleeve, thereby creating a weld between the outer diameter of the sleeve and the inner diameter of the heat exchanger tube which surrounds it. While such welding techniques have been found to create a high quality, water-tight weld joint between the ends of the sleeve and the sections of the heat exchanger tube to which they are welded, such welds also create tensile stresses in the tube due to the tendency of fused metal to draw-up upon re-solidification. If these tensile stresses are not relieved, they may promote stress corrosion cracking at this site, in the upper weld zone, thereby jeopardizing the integrity of the water-tight seal between the sleeve and the damaged section of the heat exchanger tube. While the aforementioned devices and stress relief methods can be successfully used to relieve stresses in the upper weld zones of such tubes, these prior art stress-relief techniques take up relatively large amounts of time. To understand why this is so, one must first understand some of the constraints that the working environment imposes on maintenance operations.
Because such stress-relief procedures take place within steam generators that have been operational for some time, the working environment is highly radioactive, and therefore the stress relief procedures must be remotely implemented by robotic arms such as the ROSA developed by the Westinghouse Electric Corporation. Moreover, because access to the open ends of the heat exchanger tubes can only be had through the relatively small and cramped confines of the channel head of the generator, there is, as a practical matter, room for only a single robotic arm. Since only one robotic arm is available to implement such heat treatment techniques, and since the arm must be used at all points in time to support the heater probe within the tube being heat treated, the tubes must be heat treated sequentially, with each heat treatment costing about nine minutes in time. While the device disclosed and claimed in co-pending U.S. patent application Ser. No. 213,923 might be used to simultaneously heat treat a pair of tubes, it can only do so if these tubes are adjacent to one another. Since this is often not the case, little if any savings in time can be realized by the use of such a previously known multiple probe manipulating device. As steam generator down-time can cost the electric generating utility over $500,000 a day in lost revenues, the long lengths of time necessary for the heat treatment of a number of non-adjacent tubes poses a formidable problem.
Clearly, what is needed is a system and method that is capable of simultaneously heat treating a number of non-adjacent heat exchanger tubes in a reliable and expeditious manner. Ideally, such a system should be compatible with known robotic devices, and should be simple to operate and manufacture.